The present invention relates to an exhaust gas recirculation system for an internal combustion engine very effective in reduction of MOx emission.
The conventional exhaust gas recirculation systems may be divided into a plate type and a manifold type. In the former type, exhaust gases from an exhaust tube of an internal combustion engine are recirculated through fixed restriction means into an intake tube between a carburetor and a throttle valve in such a way that the volume of recirculated exhaust gases may be a function of a pressure in the exhaust tube. In the latter type, exhaust gases from the exhaust tube are recirculated through a restriction means into an intake manifold. Both types of the external exhaust gas recirculation systems are very effective in reduction of NOx emission in exhaust gases.
In addition to the external exhaust gas recirculation systems, it has been found that residual gases which remains within the cylinders as a result of the incomplete scavenging have the ability of reducing NOx emission comparable to the external exhaust gas recirculation systems. Therefore both the external exhaust gas recirculation and the residual gases (to be referred to as "internal exhaust gas recirculation") must be taken into consideration in the reduction of NOx and must be optimumly controlled for the attainment of the maximum NOx reduction.
To this end, it is an ideal method to maintain constant the ratio of the volume of the total exhaust gas recirculation (external exhaust gas recirculation + internal exhaust gas recirculation) to the volume of intake air. However, the ratio of the internal exhaust gas recirculation becomes low with a heavy load and high with a light load. Accordingly, the ratio of the external exhaust gas recirculation must be controlled depending upon the load on the engine in such a manner that the ratio of the external exhaust gas recirculation is increased with a heavy load and reduced with a light load.
With the prior art plate type exhaust gas recirculation system, the ratio of the volume of exhaust gases to be recirculated to the volume of intake air can be maintained at a predetermined level, but cannot be varied depending upon the variations in load. Furthermore the plate type exhaust gas recirculation system results in the adhesion of foreign matters to the throttle valve, adverse thermal effects on the carburetor and the icing problem at low temperatures.
In the manifold type exhaust gas recirculation system, the exhaust gases are directly recirculated into the intake manifold so that the adverse effects as encountered in the plate type exhaust gas recirculation system may be avoided. However in the prior art manifold type exhaust gas recirculation system, the volume of exhaust gases to be recirculated is controlled depending upon the negative intake pressure or the negative pressure at a venturi so that the volume of exhaust gases recirculated is dependent upon the difference between the back pressure and the intake pressure and the opening area of restriction means. As a result, the recirculation volume is high with a light load while low with a heavy load especially under the influence of the negative pressure of intake air. That is, the control on the volume of exhaust gases to be recirculated is contrary to the ideal control described above. For this reason with a light load, surging and misfiring result, and with a heavy load NOx emission cannot be reduced to an acceptable degree.
To overcome these problems there has been proposed a modified manifold type exhaust gas recirculation system wherein an exhaust gas recirculation control valve which is responsive to the negative intake pressure which in turn is controlled by a modulator is disposed within an exhaust gas recirculation passage so that the ratio of the volume of externally recirculated exhaust gases to the volume of intake air may be maintained constant. However as with the plate type exhaust gas recirculation system this modified system still cannot control the external exhaust gas recirculation ratio in response to the variations in load.